An optical delay line can be used to separate the particular colors, i.e. the light waves of different wavelengths, of a chromatic spectrum. An example of such a device is an arrayed waveguide grating (AWG). If a conventional AWG is designed for extremely high color or spectral resolution, the AWG must have great physical size, which makes practical application of such devices difficult.
Such AWGs are commonly used as optical demultiplexers in wavelength division multiplexed (WDM) systems. The transmission capacity of such WDM systems is dependent on the AWG's ability to separate out the different wavelength carriers of the optical network. The higher the spectral resolution, the more information that will be able to be transmitted along the single optical fiber.
One such optical delay line that provides a very high resolution AWG system, i.e., a hyper-resolution AWG system is described in U.S. Pat. No. 7,043,108 to Olsen, entitled PLANAR MULTIPLE-TAPPED OPTICAL DELAY LINE, the teachings and disclosure of which are incorporated by reference in their entireties herein. As described therein, such a delay line includes a spiral of optical waveguide having a number of spiral waveguide loops disposed within a single plane. Broadband optical energy is received at an input end of the waveguide spiral and is released through optical tap areas in the waveguide loops. The end of the waveguide can be a waveguide termination such as a matched load that decreases reflection. Olsen describes that while waveguide loop numbers on the order of 10 are suitable for many purposes, waveguide loop numbers on the order of 100 will provide greater frequency resolution fidelity. The minimum curvature of the waveguide loops will be dictated by the bending limits suggested by the waveguide manufacturer.
In an understood manner, the length by which optical signals travel in a waveguide affects the phase of the traveling light and hence provides a mechanism by which the colors of the incoming light can be separated. In essence, different waveguide phase lengths permit different frequencies of light to be segregated from other frequencies of light. It is thereby possible to spread the colors of light out by creating a phase length difference that corresponds to a particular desired travel time of the light, wherein 1 divided by this desired travel time creates the approximate upper limit to the frequency resolution of a hyper-resolution AWG.
For example, to create a frequency resolution of approximately 100 MHz (0.1 GHz), a path length (in a vacuum) of approximately 10 feet of travel or 10 nanoseconds of light travel time is required. Because the index of refraction of glass (waveguide) differs from that of a vacuum, a shorter length of fiber is suitable to accomplish this delay. In this instance, approximately six feet of waveguide, between the first and last optical taps provides a 0.1 GHz resolution. To create a 1 GHz frequency resolution, approximately 0.6 feet of optical waveguide between the first and last optical taps is needed. To create a 10 GHz frequency resolution, approximately 0.06 feet of optical fiber between the first and last optical taps is needed. Higher resolution can be achieved by lengthening the distance between these first and last optical taps.
Unfortunately, for proper operation of the AWG, each loop must have a path length that is an integer multiple of some fixed path length, i.e. Li+1−Li=ΔL, where Li is the length of the ith path, equivalently Li−L1=(i−1)ΔL. This requirement has heretofore dictated the physical size of the high resolution (<100 MHz spectral resolution) AWG photonic frequency separation devices.
There is thus a need for an optical delay line that provides high spectral resolution but that is compact. For many applications it is desirable that such a delay line be realizable in planar form. An optical delay line having these attributes can enable a very high resolution AWG system (a hyper-resolution AWG system) with spectral resolution <100 MHz. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of embodiments of the invention provided herein.